The present disclosure generally relates to a disk-shaped filter cartridge having filter medium having two or more filtration zones or layers of different particle-retention capability (“PRC”) with respect to the fluid suspension which is filtered under ambient filtration conditions (“composite medium”) wherein the zones or layers are positioned with respect to one another such that the contaminant-holding capacity (“CHC”) per unit area of the composite medium is greater than the contaminant-holding capacity (“CHC”) per unit area of the filtration zone or layer of the composite medium which has the greatest PRC when such is extrapolated to the depth of the composite medium and includes a new and innovative edge seal. More particularly, one representative embodiment of the present disclosure relates to a disk-shaped filtration unit employing such filter medium wherein the zones or layers are positioned with respect to one another such that medium layer most remote from the separator has a relatively larger PRC than a second medium contiguous therewith, any additional mediums positioned between the second medium and the separator have a pore size smaller than the most remote medium layer but which may be larger than the second medium layer, wherein the disk-shaped filter unit includes a new and innovative edge seal. Yet more particularly, another representative embodiment of the present disclosure relates to a disk-shaped filter unit having upper and lower composite medium separated by a separator layer wherein the zones or layers of each composite medium are positioned with respect to one another such that the CHC per unit area of the composite medium is greater than the CHC per unit area of the filtration zone or layer of the composite medium which has the greatest PRC when such is extrapolated to the depth of the composite medium wherein the operable upper and lower composite medium are operatively connected by an innovative edge seal. More specifically, yet another representative embodiment of the present disclosure relates to a disk-shaped filter unit having an upper composite medium and a lower composite medium separated by a non-filtering separator layer, wherein each of the filter medium is comprised of two or more zones or layers of filter material of the same or different composition and/or fabrication, each layer being positioned with respect to one another such that the more distal the zone or layer from the separator layer the lesser the PRC with respect to the fluid suspension which is to be filtered under attendant filtration conditions and the composite mediums are connected by an innovative edge seal. Still another representative embodiment of the present disclosure relates to a lenticular filtration unit having an upper composite medium and a lower composite medium, separated by a non-filtering separator layer, wherein each composite medium is fashioned to have a graded PRC in the direction of flow such that, as positioned on the non-filtering separator layer, the lenticular filtration unit is capable of more efficiently retaining smaller and smaller particles as the fluid moves from the surface of the filter medium towards the non-filtering separator layer and the composite mediums are operatively connected by an innovative edge seal wherein the innovative edge seal in combination with the upper and lower composite mediums provide a filtration unit having a very low tendency toward medium buckling resulting from the inclusion therein of a thermoplastic edge seal having certain desirable mechanical characteristics.
Disk-shaped filtration units are well known in the art, and have conventionally comprised two overlying similarly-shaped filter medium separated from one another along the majority of their opposing surface areas by a non-filtering separator element, and affixed to one another along their perimeter edges by an edge seal. Conventionally, the filter medium and the separator element each have a central void of about the same shape and dimension such that a uniform through bore in the filtration unit is formed when each void is aligned.
The separator element is conventionally composed of a material distinct from the composition of the medium which abuts the separator, and generally has openings therein of such size that the separator is substantially non-filtering with respect to the material to be filtered given its position within the cell-type filtration unit. In addition to separating the two filter medium, and supporting the filter medium under differential pressure, the separator element is generally fashioned to have a plurality of conduits formed therein, such conduits communicating with the central void of the separator and the through bore of the filtration unit to allow flow to get from the outer-diameter or edge of the disk-shaped filtration unit to a stacked common bore. Separators are conventionally fashioned from polymeric materials, in particular plastics, although they can also be fashioned from other materials, such as, for example, metals, ceramics and other material, as are in known in the art to be capable of separating the two layers effectively in a particular filter application environment.
A separator element may be manufactured to include upper and lower ribs of varying thickness to maintain the medium in a disk-shape. Lenticular filtration units, comprising two disk-shaped filter medium separated by a closed-curve non-filtering separator element, are particularly common place in the art. Separators used in lenticular filters generally have a plurality of ribs extending radially outward from a central aperture in a spoke-like fashion. An example of a lenticular cell-type filtration unit is found in U.S. Pat. No. 4,783,262 to Ostreicher et al., the disclosure of which is herein incorporated by reference to the extent not inconsistent with the present disclosure. However, any rib geometry may be used in the separator design to effectively maintain separation of the two disk-shaped filter medium and provide a flow channel from the outer diameter or edge of the disk-shaped filtration unit to the stacked common bore.
Generally, the outer circumference of the two medium discs of a lenticular filtration unit are held together by an insert molding process which encapsulates the circumferences in plastic. U.S. Pat. No. 4,347,208 to Southall, the disclosure of which is herein incorporated by reference to the extent not inconsistent with the present disclosure, discloses a method of making a filtration unit having a sealed periphery which includes the steps of placing two medium discs, and interposed separator, into a mold and injecting a thermoplastic polymer into the mold to form a seal around the two medium discs. The Southall patent discloses polypropylene, polyethylene, nylon, and polysulfone as the preferred thermoplastic polymers for molding the edge seal.
Lenticular filtration units use a variety of materials for filtering fluids, including without limitation, glass fibers, diatomaceous earth, perlite, cellulose, and binder resins. The filter medium is typically produced by a wet laid papermaking operation. Medium thickness generally ranges between about 0.130 to about 0.218 inches depending on the material formulation. By “filter medium,” it is meant a porous article or mass having a porosity, or carrying/producing a charge, or incorporating matter which binds matter in the suspension, such that it will separate out matter in suspension in the fluid, gas or liquid, which is to be filtered.
Lenticular filtration units generally have a through bore and are generally employed in conventional practice by stacking one on another in seriatim to form a common bore, such common bore communicating with one or more separator conduits. The stacked filtration unit assembly, or filter cartridge, is then enclosed in a housing having an inlet port and an outlet port, the common bore typically being positioned in the housing so as to communicate with the outlet port. Not infrequently, fluid is supplied to the housing at high temperature and/or high pressure. The fluid enters the gaps between the adjacent filtration units and then passes through the filter medium covering the separator. As the fluid passes through the filter medium, undesirable materials such as aggregates and particulates are removed from the fluid. The filtered fluid then flows along the conduits of the separator to the common bore and exits the housing via the outlet port.
A significant advantage of stacked filter cartridges over other filtration systems is that the surface area of the filter material is quite large when compared to the total volume displaced by the stacked filter cartridge. This relatively large surface area permits larger volumes of fluid to be filtered, as compared to cartridges displacing a similar volume but which have a lower surface area, over the same period of time. Conventional stacked filter cartridges are useful in a variety of applications, including the filtration of fluids such as beverages, dielectric oils, chemicals, etc. Filter cartridges find use as both primary filters and pre-filters.
When used as pre-filters, stacked filter cartridges may be located upstream from another stacked filter cartridge, or from a filter cartridge of dissimilar construction, e.g. a pleated membrane filter. Owing to their large available surface area, disk-shaped filter cartridges are frequently used to remove particulates from a fluid stream prior to microfiltration by a membrane filtration unit. The pre-filter is designed to remove particulates which would otherwise plug the membrane of the membrane filtration unit, thereby preventing the reduction of both the filtration flow rate (or at constant flow, increasing the pressure differential through the membrane of the membrane filtration unit) and the reduction of the life of the membrane used by the membrane filtration unit.
While such dual filtration systems result in a highly purified effluent, the cost involved in maintaining both the pre-filter and qualifying filtration units is relatively high. Additional operational costs are incurred in using multiple filtration units in that additional housings which must be purchased and installed to incorporate each succeeding filtration unit. Further, there is a downtime cost with respect to the replacement of the filters of either filtration units, the filters of one filtration unit not infrequently being optimally replaced at a different time than the filters of other filtration unit.
While multi-layer cell-type filtration units are known in the art, additional layers serve purposes other than to increase CHC. For example, Cuno 05UW Zeta-Plus® is constructed of two identical cellulose and glass fiber layers (having the same pore size distribution and charge potential, as well as the same CHC per unit area and PRC) having a water absorbent layer of different materials located there-between. The water absorbent layer is interposed to remove water from an oil filtrate and does not act as a particulate filtration medium. The cellulose layers act both as particle retention filters and as support for the relatively weak water absorbent layer as the water absorbent layer swells. A filter of similar construction is also produced commercially by Alsop®. Zeta-Plus® filters are also available having a layer of spunbond polypropylene or polyester non-woven placed between the separator and the cellulosic filter medium. The interposed layer does not act as a filter medium, but rather is used to support the filter medium, in particular under differential pressure. Zeta-Plus® filters having a layer of spunbond or netting placed on the outer surface of the filter medium are also known. Such outer layer is used to provide support in a reverse flow/pressure condition and helps insure that fluid flow is not obstructed between cells if the medium faces of two adjacent cells are in contact. Flowtech® also produces a similar commercial product. In neither case does the outer layer act as a filter medium.
A multi-layered construction is also found in the Roki Techno ABSO-AB® Series lenticular filters. In this product, two cellulosic filter medium layers are disposed on each side of the separator. One thin layer of melt-blown material, of about half the thickness of the overlying cellulosic filter medium, is located under the two-layer cellulosic filter medium, in contact with the separator—that is the melt-blown material is located between the separator and inner cellulose medium layer. The melt-blown material layer is used to reduce medium migration from the cellulosic filter medium to the separator. Such melt-blown material layer does not increase particle retention over the cellulosic filter medium. The melt-blown material layer, as measured by a Coulter Porometer, has a 12½ micron mean flow pore size versus 2–4 micron mean flow pore size for the cellulose filter medium.
Japanese Utility Model 5-2709 also discloses a multi-layer lenticular filtration unit but does not appear to describe the particle retention properties of the layers. No disclosure, teaching or suggestion is believed made by Japanese Utility Model 5-2709 that incorporates filter medium having two or more layers and/or zones of different PRC, with respect to the fluid suspension, which is filtered, under attendant ambient filtration conditions.
As mentioned above, in prior art filters of this type, the two medium discs are joined together by a thermoplastic edge seal, which grips the two medium layers to form a liquid tight seal at the outer periphery of the unit. As also mentioned above, U.S. Pat. No. 4,347,208 to Southall discloses a method of making a filtration unit having a sealed periphery which includes the steps of placing two medium discs and an interposed separator into a mold and injecting a thermoplastic polymer into the mold to form a seal around the two medium discs. The Southall patent discloses that polypropylene, polyethylene, nylon, and polysulfone are the preferred thermoplastic polymers for molding the edge seal, all of which have a relatively high elastic modulus.
It is known that thermoplastic edge seals formed by injection molding undergo radial shrinkage when cooled after solidification. This shrinkage apparently induces a compressive radial stress in the filter medium, thus increasing the tendency towards buckling the filter medium when the lenticular filter is in use, a condition wherein the filter medium layers deflect in a vertical plane. Furthermore, conventionally such thermoplastic edges are molded around a dry, solid, medium discs, resulting in a thermoplastic ring diameter larger than if thermoplastic material was molded and then allowed to cool unrestrained. Therefore, the restraint from typical shrinkage during molding induces molded-in stress in the thermoplastic edge seal, causing a distortion of the filter unit during initial heat sterilization/sanitization cycles. The Southall patent recognizes that this is a characteristic of thermoplastic materials and discloses that the amount of polymer used during the injection molding process should be kept to a minimum to prevent excessive radial shrinkage of the edge seal and thereby decrease the tendency towards buckling.
It is well known in the art that the problem of lenticular filtration unit buckling is exacerbated during hot wash cycles in which lenticular filtration units are subjected to temperatures in excess of about 180° F. During hot wash cycles, as the filter assemblies are heated and the filter medium wet out, differential expansion coefficients between the thermoplastic components of the lenticular filtration unit and the lenticular filtration medium result in increased compressive stresses in the medium elements, further increasing the tendency towards buckling. During the period of time following a hot wash cycle, commonly referred to as the post flush period, the thermoplastic edge seal cools to room temperature and thus contracts radially inwardly while the cellulosic lenticular medium elements, still wet, remain swollen in an expanded state. Consequently, there is an interference generated and an increased tendency towards buckling due to the incompatibility of the edge seal material and the material of the filtration medium.
Thus, there is, therefore, a need for a more economical filtration system that results in decreased down time due to filter replacement and to provide for highly purified effluent without the need to resort to a dual filter filtration system. Further, it is desirable that the useful life of any qualifying filter used in a process be extended. Still further, it would be beneficial, therefore, to provide a lenticular filtration unit with a molded thermoplastic edge seal which does not impose excessive compressive forces on the lenticular filtration medium once cooled after injection molding, during hot wash cycles, or during post flush periods.